Process for improving batch annealed strip surface quality

ABSTRACT

A process is described for eliminating surface discoloration of batch annealed steel products, where said discoloration has been described as graphite staining. Said process is characterized by an annealing step which includes the introduction of a small quantity of CO2 to the annealing atmosphere comprising hydrogen and nitrogen.

United States Patent [191 Fisher et a].

[ 1 Mar. 25, 1975 PROCESS FOR IMPROVING BATCH ANNEALED STRIP SURFACEQUALITY [75] Inventors: Thomas W. Fisher, Bethlehem;

EdwardD. Melcher, Whitehall, both of Pa.

[73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: Nov. 21, 1973 [21] App]. No.: 417,852

[52] US. Cl. 148/16.7, 148/16 [51] hit. C1. C21e 1/74, C210 9/46 [58]Field of Search 148/12.1, 16, 16.7

[56] References Cited UNITED STATES PATENTS 1,118,275 11/1914 Fraschl48/l6.7 X

2,085,597 6/1937 Marshall l48/16.7 2,304,518 12/1942 Williams l48/l2.l2,402,013 6/1946 Billeter et al. l48/16.7

Primary Examiner-C. Lovell Attorney, Agent, or Firm-Joseph J. OKeefe;William B. Noll [57] ABSTRACT A process is described for eliminatingsurface discoloration of batch annealed steel products, where saiddiscoloration has been described as graphite staining. Said process 'ischaracterized by an annealing step which includes the introduction of asmall quantity of CO to the annealing atmosphere comprising hydrogen andnitrogen.

3 Claims, No Drawings PROCESS FOR IMPROVING BATCH ANNEALED STRIP SURFACEQUALITY CROSS-REFERENCE TO RELATED APPLICATION This application is acompanion case to Ser. No. 417,851 filed concurrently with thisapplication and assigned to the assignee herein, entitled Black PlateSteel and Method.

BACKGROUND OF THE INVENTION This invention is directed to a process forimproving batch annealed strip surface quality, more particularly to theannealing process wherein a washed steel strip in coil form is subjectedto an atmosphere comprising nitrogen, hydrogen and a small quantity ofcarbon dioxide.

While this invention has particular utility in the production of tinplate,.it is not intended to be so restricted as it is applicable toother steel products where-surface quality is critical. However, forconvenience, the discussion to follow will be directed primarily to theprocessing of steel for use as tin plate, more specifically, theprocedures leading to final electrolytically tinning.

Generally, the processing of the steel follows a conventional practiceresulting in a hot-mill coil as thin as about 0.065 inch or as thick asabout 0.125 inch. From the hot-mill, the coil is subjected to a picklingtreatment to remove the hot-mill scale and oxide. To facilitate thesevere cold reduction of the strip, which may run as high as 90%, alubricant or oil is applied to the surfaces thereof. The cold-reducedsteel strip, which is generally quite hard, must be softened byannealing, i.e., batch or continuous. Following the anneal, the steelstrip may be temper rolled to insure the proper degree of flatness andimprove surface quality for the subsequent tinning operation; or thestrip may be further reduced, prior to timing, to impart certain certaindesirable metallurgical and physical properties.

Heretofore, certain types of surface quality problems were noted withbatch annealed steel and much of the effort by the prior art wasdirected at the solving thereof. One of said problems was the appearanceof a carbon edge or snakey-edge. A second problem, the solution of whichis the discovery herein, was the appearance of a relatively continuousor patchy staining of the sheet or strip surface. Through thediscoveries herein, the latter problem has been defined, forconvenience, as graphite staining. With one or both of said problems,any attempt to apply a tin coating resulted in relatively poor adherenceor substandard corrosion protection and non-uniform appearance.

The first of said problems has long been recognized I by the steelindustry, and the approaches and/or theories postulated to solve it havefollowed various paths leading to different conclusions with sometimesindeterminate results. For instance, the different approaches were inpart a result of variations in the processing sequence described above.Very often the coldrolled steel strip was box annealed with the oil orlubricant still on the surfaces thereof. To such practitioners, theappearance of carbon edge was attributed to the breakdown or cracking ofthe lubricants used for rolling which, as indicated above, are presenton the surfaces thereof during anneal.

Others postulated that the problem arises during annealing from thecatalytic breakdown of carbon containing gases, and that the primarycatalysts for this breakdown are the presence on the steel surface ofminute iron particles dislodged during rolling. U.S. Pat. No. 3,725,l4lto Hill presents additional background as to the problems and theattempts by the prior art to solve them. A common factor of eachapproach was the assumption that carbon deposition caused carbon edge orsnakey-edge, and that their attempts were directed at means to eliminatesaid deposition.

The present invention evolved as a result of the recognition that thetwo problems are totally different and that the mechanism involvedtherein are different. It is now believed that graphite staining is atleast in part the result of carbon diffusing from within the steel tothe strip surface. As will be explained hereinafter, the presentinvention deals with the annealing of a washed steel strip which has'hadsubstantially all of the oil and lubricants removed therefrom. Obviouslythen there can be no breakdown or cracking of the lubricants during theanneal. In any case the theoretical approach and solutions herein arenot found in the prior art.

SUMMARY OF THE INVENTION This invention relates to a method foreliminating graphite staining of cold rolled and washed steel subjectedto a process anneal. The improved surface quality thereof is achieved bya procedure which includes batch annealing said steel at temperaturesbetween about llOO F. to l300 F. in a non-oxidizing atmospherecomprising hydrogen, nitrogen and about 3 to about 9%, by volume, carbondioxide, preferably between about 4 to about 8% carbon dioxide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT In the preferred practiceof this invention, a tightly wound coil of steel strip or a plurality ofstacked coils are placed on a generally raised base of a conventionalfurnace with the coil axes vertical. An inner cover is placed over thecoils and sealed about said base such as by sand or other means wellknown in the art. An annealing furnace hood is then placed over saidinner cover in preparation for the annealing cycle. Heating means, whichmay be radiant heating tubes or other conventional heating mechanism,also form part of the annealing furnace along with suitable conduitsconnected thereto through the raised base. By means of the latter theair is purged from the inner cover permitting the introduction of aprotective atmosphere, the criticality and nature of which will becomeapparent hereinafter.

Before discussing the annealing cycle, it will be appreciated from theinitial discussion that various practices are followed in the processingcycle. No attempt will be made to explain why certain practices arefollowed except to say that the particular problems noted herein are notsignificant or even apparent in some of said practices. For example,where a continuousannealing practice is followed, i.e., rapid heatingwith complete bathing of surfaces in annealing atmosphere, the saidsurface problems are not apparent.

For the batch annealed products, both tight and open coils may beannealed. With the latter, spacers are provided between adjacentconvolutions of the coil thus permitting the annealing atmosphere toreach the inner portions of the said coil. Where such a practice isfollowed, graphite staining is not a significant problem. Hence, it isonly where tight coils are batch annealed that such problems becomeserious concerns to the operators. For these reasons, the presentinvention is directed to a batch or process anneal of tight coils.

The evidence or existence of the problems noted herein are not such ascan be measured quantitatively or against a standard. Graphite staining,as well as carbon edge, are evident from visual inspection. The latteris characterized by a wavy band, varying in degrees of darkness, alongthe outside of the strip. Experience has shown that such a conditionaffects the coatability of the steel, whether it be metallic coated,such as electrolytic tinning, or coated with a non-metallic, i.e.,bonderized, painted, or thermally cured resinous coating. In any event,graphite staining appears as dull gray patches or may be relativelycontinuous throughout the entire strip width of the strip surface. Withgraphite staining, there is not only a problem with coating appearance,but difficulties arise if a subsequent temper rolling or cold reductionis desired.

With this explanation, consideration may now be given to the details ofthe annealing cycle. l-leretofore, the conventional annealing cycleemployed an atomosphere comprising a mixture of hydrogen and nitrogencontaining between about 4% .to 25% hydrogen and the remainder nitrogen.

The present invention represents an improvement thereover in that theatmosphere is further limited such that to the standard HN gas there isintroduced a quantity of carbon dioxide between about 3% to 9%.Specifically, the annealing atmosphere of this invention comprises anon-oxidizing mixture of gases consisting essentially of, by volume:

To demonstrate the effectiveness of the above described atmosphere ineliminating serious graphite staining, stacked sheet samples weresubjected to various annealing atmospheres. The steels chemistry isreported in TABLE 1, followed by the observed results in TABLE ll.Procedurally, cold rolled full hard steel sheets, having the chemistrylisted in TABLE I and with the tandem mill oil still on the surfacesthereof, were cut into 4 inch x 8 inch panels. To remove substantiallyall of said oil, the panels were washed by dip cleaning and scrubbing ina 190 F. alkaline cleaning solution (PENNWALT 30 ORTHOSlL, a trademarkof Pennwalt Corporation). After cleaning, the panels were rinsed andblown dry with compressed air. To simulate a tight coil, a number ofpacks of about 30 panels were assembled by stacking the panels on top ofone another. The stacks were held tight by compressing each betweenoversized stainless steel plates. Each stack or pack was drawn tightlytogether by stainless bolts passing through the outer edges of thestainless plates and alongside the stacked steel panels within.

The pack assembly was then placed in a stainless steel tank or cover andwelded shut. The desired annealing atmosphere was fed into the tankthrough an inlet nozzle, circulated thereabout, and permitted to exitvia an outlet nozzle. Additionally, a thermocouple was inserted throughthe outlet nozzle to monitor the temperature inside the tank. The tank,with gas lines and thermocouple attached, was placed in a furnace andthe annealing cycle begun. The temperature of the furnace and tank wasraised to about l200 F. and the panels soaked for about 10 hours. At theend of the soak, the tank was removed from the furnace and allowed tocool with the gas lines still connected feeding the desired atmosphere.When the tank was completely cooled down, it was opened and the panelsremoved and examined.

TABLE I CHEMlSTRY Steel C Mn P S (u Sn A .08 .27 .004 .004 .014 B .08.32 .005 .013 .012 .002 C .09 .46 .005 .030 .0l4 .004

TABLE ll RESULTS Test Atmosphere, Vol. 7: Graphite Carbon Series H N COStaining Edge Al 4 96 Heavy No Bl 4 96 Heavy No Cl 4 96 Very No Light A24 92.6 3.4 Variable No Degrees B2 4 92.6 3.4 Very No Light C2 4 92.6 3.4No No A3 3.6 89.6 6.8 No No B3 3.6 89.6 6.8 No No C3 3.6 89.6 6.8 No NoA4 3.7 86.8 9.5 No Yes B4 3.7 86.8 9.5 No Yes C4 3.7 86.8 9.5 No Yes A517 76 7 No No B5 17 76 7 No No C5 17 76 7 No No It is clear from theresults above that the steel panels, irrespective of chemistry, weresuccessfully treated in the preferred atmosphere of this invention. Asthe annealing atmosphere approached the leaner portion of the range(series 2) some graphite staining was observed in the low maganesesteels. For a more detailed discussion on the effects of composition incontrolling graphite staining, reference is made to said copendingapplication. Finally, when the atmosphere was too high in CO a carbonedge began to appear. It would thus appear that carbon edge can readilybe controlled by minimizing the quantity of carbon dioxide in theannealing atmosphere. However, experience has shown that when stackedcoils are annealed in a commercial DX gas, nominally containing about10% CO, 5% CO 15% H balance N some carbon edge and graphite stainingwill appear. Thus, while small amounts of CO might be tolerated in theannealing atmosphere, it is obvious that significant amounts of C0 aredetrimental to the attainment of the results desired herein.

In any case, by the procedure described herein, an effective means istaught for eliminating the problem of graphite staining.

We claim:

1. A process of annealing a coil of steel strip, where adjacentconvolutions thereof are in contact with one another, comprising thesteps of placing said coil in an carbon dioxide is present in an amountbetween about 4% to about 8%. siTh'gpioce according to claim 1 whereinsaid coil is soaked at the temperature between about 1 l00 F. to aboutl300 F., and said non-oxidizing gas is nmintained for at least the timeduring the soaking period. l l

1. A PROCESS OF ANNEALING A COIL OF STEEL STRIP, WHERE ADJACENTCONVOLUTION THEREOF AREIN CONTACT WITH ONE ANOTHER COMPRISING THE STEPSOF PLACING SAID COILL IN AN ENCLOSURE INTRODUCING INTO SAID ENCLOSURE ANON-OXIDING GAS CONSISTING OF, BY VOLUME, HYDROGEN IN AN AMOUNT BETWEENABOUT 45% TO ABOUT 25%, ABOUT 3% TO ABOUT 9% CARBON DIOXIDE, BALANCENITROGEN, AND HEATING SAID COIL IN SAID NON-OXIDIZING GAS TO ATEMPERATURE BETWEEN ABOUT 1100*F. TO ABOUT 1300*F.
 2. The processaccording to calim 1 wherein said carbon dioxide is present in an amountbetween about 4% to about 8%.
 3. The process according to claim 1wherein said coil is soaked at the temperature between about 1100* F. toabout 1300* F., and said non-oxidizing gas is maintained for at leastthe time during the soaking period.